geologic fault


geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the processes by which th ...

, a fault is a
planar Planar may refer to: Science and technology * Planar (computer graphics) In computer graphics, planar is the method of arranging pixel data into several '' bitplanes'' of RAM. Each bit in a bitplane is related to one pixel on the screen. Unlike ...
fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...

or discontinuity in a volume of
rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in w ...
across which there has been significant displacement as a result of rock-mass movements. Large faults within the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...

's crust result from the action of
plate tectonic upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate tectonics (from the la, label=Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the written ...
forces, with the largest forming the boundaries between the plates, such as
subduction zones Subduction is a geological process in which the oceanic lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On ...

subduction zones
transform fault A transform fault or transform boundary, sometimes called a strike-slip boundary, is a fault Fault commonly refers to: *Fault (geology), planar rock fractures showing evidence of relative movement *Fault (law), blameworthiness or responsibility ...

transform fault
s. Energy release associated with rapid movement on
active fault An active fault is a fault Fault commonly refers to: *Fault (geology), planar rock fractures showing evidence of relative movement *Fault (law), blameworthiness or responsibility Fault(s) may also refer to: Arts, entertainment, and media * "Fau ...
s is the cause of most
earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that ...

s. Faults may also displace slowly, by
aseismic creep in 2003. It was demolished in 2009., 200x200px In geology, aseismic creep or fault creep is measurable surface displacement along a Geologic fault, fault in the absence of notable Earthquake, earthquakes. Aseismic creep may also occur as "after-slip ...
. A ''fault plane'' is the
plane Plane or planes may refer to: * Airplane or aeroplane or informally plane, a powered, fixed-wing aircraft Arts, entertainment and media *Plane (Dungeons & Dragons), Plane (''Dungeons & Dragons''), a location in the multiverse *Plane (Magic: Th ...
that represents the fracture surface of a fault. A ''
fault trace 300px, Along the Motagua Fault trace (1976 Guatemala earthquake) where it crosses the Gualán">1976_Guatemala_earthquake.html" ;"title="Motagua Fault trace (1976 Guatemala earthquake">Motagua Fault trace (1976 Guatemala earthquake) where it crosses ...
'' or ''fault line'' is a place where the fault can be seen or mapped on the surface. A fault trace is also the line commonly plotted on
geologic map A geologic map is a special-purpose map made to show various geological features. Rock units or geologic strata are shown by color or symbols. Bedding planes and structural features such as faults, folds, are shown with strike and dip St ...
s to represent a fault. A ''fault zone'' is a cluster of parallel faults. However, the term is also used for the zone of crushed rock along a single fault. Prolonged motion along closely spaced faults can blur the distinction, as the rock between the faults is converted to fault-bound lenses of rock and then progressively crushed.

Mechanisms of faulting

Owing to
friction Friction is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related en ...

and the rigidity of the constituent rocks, the two sides of a fault cannot always glide or flow past each other easily, and so occasionally all movement stops. The regions of higher friction along a fault plane, where it becomes locked, are called '' asperities''. Stress builds up when a fault is locked, and when it reaches a level that exceeds the
strength Physical strength *Physical strength, as in people or animals *Hysterical strength, extreme strength occurring when people are in life-and-death situations *Superhuman strength, great physical strength far above human capability *A common attrib ...
threshold, the fault ruptures and the accumulated
strain energy In physics, the elastic potential energy gained by a wire during elongation with a stretching force is called strain energy. For linearly elastic materials, strain energy is: U = \frac 1 2 V \sigma \epsilon = \frac 1 2 V E \epsilon^2 = \frac 1 2 ...
is released in part as
seismic wave Seismic waves are waves of energy In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its M ...
s, forming an
earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that ...

. Strain occurs accumulatively or instantaneously, depending on the
liquid state A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers to a fluid flow, flow in which the material density is constant within a fluid parc ...

liquid state
of the rock; the
ductile Ductility is a mechanical property commonly described as a material's amenability to Drawing (manufacturing), drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation ...
lower crust and Mantle (geology), mantle accumulate deformation gradually via Shear (geology), shearing, whereas the brittle upper crust reacts by fracture – instantaneous stress release – resulting in motion along the fault. A fault in ductile rocks can also release instantaneously when the strain rate is too great.

Slip, heave, throw

''Slip'' is defined as the relative movement of geological features present on either side of a fault plane. A fault's ''sense of slip'' is defined as the relative motion of the rock on each side of the fault concerning the other side. In measuring the horizontal or vertical separation, the ''throw'' of the fault is the vertical component of the separation and the ''heave'' of the fault is the horizontal component, as in "Throw up and heave out". The vector of slip can be qualitatively assessed by studying any drag folding of strata, which may be visible on either side of the fault. Drag folding is a zone of folding close to a fault that likely arises from frictional resistance to movement on the fault. The direction and magnitude of heave and throw can be measured only by finding common intersection points on either side of the fault (called a piercing point). In practice, it is usually only possible to find the slip direction of faults, and an approximation of the heave and throw vector.

Hanging wall and footwall

The two sides of a non-vertical fault are known as the ''hanging wall'' and ''footwall''. The hanging wall occurs above the fault plane and the footwall occurs below it. This terminology comes from mining: when working a tabular ore body, the miner stood with the footwall under his feet and with the hanging wall above him. These terms are important for distinguishing different dip-slip fault types: reverse faults and normal faults. In a reverse fault, the hanging wall displaces upward, while in a normal fault the hanging wall displaces downward. Distinguishing between these two fault types is important for determining the stress regime of the fault movement.

Fault types

Faults are mainly classified in terms of the angle that the fault plane makes with the earth's surface, known as the Strike and dip, dip, and the direction of slip along the fault plane. Based on the direction of slip, faults can be categorized as: * ''strike-slip'', where the offset is predominantly horizontal, parallel to the fault trace; * ''dip-slip'', offset is predominantly vertical and/or perpendicular to the fault trace; or * ''oblique-slip'', combining strike-slip and dip-slip.

Strike-slip faults

In a strike-slip fault (also known as a ''wrench fault'', ''tear fault'' or ''transcurrent fault''), the fault surface (plane) is usually near vertical, and the footwall moves laterally either left or right with very little vertical motion. Strike-slip faults with left-lateral motion are also known as ''sinistral'' faults and those with right-lateral motion as ''dextral'' faults. Each is defined by the direction of movement of the ground as would be seen by an observer on the opposite side of the fault. A special class of strike-slip fault is the
transform fault A transform fault or transform boundary, sometimes called a strike-slip boundary, is a fault Fault commonly refers to: *Fault (geology), planar rock fractures showing evidence of relative movement *Fault (law), blameworthiness or responsibility ...

transform fault
when it forms a plate tectonics, plate boundary. This class is related to an offset in a spreading center, such as a mid-ocean ridge, or, less common, within continental lithosphere, such as the Dead Sea Transform in the Middle East or the Alpine Fault in New Zealand. Transform faults are also referred to as "conservative" plate boundaries since the lithosphere is neither created nor destroyed.

Dip-slip faults

Dip-slip faults can be either normal ("extensional fault, extensional") or reverse. In a normal fault, the hanging wall moves downward, relative to the footwall. A downthrown block between two normal faults dipping towards each other is a graben. An upthrown block between two normal faults dipping away from each other is a Horst (geology), horst. Low-angle normal faults with regional tectonics, tectonic significance may be designated detachment faults. A reverse fault is the opposite of a normal fault—the hanging wall moves up relative to the footwall. Reverse faults indicate compressive shortening of the crust. The Strike and dip, dip of a reverse fault is relatively steep, greater than 45°. The terminology of "normal" and "reverse" comes from coal mining in England, where normal faults are the most common. A thrust fault has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°. Thrust faults typically form ramps, flats and fault-bend (hanging wall and footwall) folds. Flat segments of thrust fault planes are known as ''flats'', and inclined sections of the thrust are known as ''ramps''. Typically, thrust faults move ''within'' formations by forming flats and climb up sections with ramps. Fault-bend folds are formed by the movement of the hanging wall over a non-planar fault surface and are found associated with both extensional and thrust faults. Faults may be reactivated at a later time with the movement in the opposite direction to the original movement (fault inversion). A normal fault may therefore become a reverse fault and vice versa. Thrust faults form nappes and klippen in the large thrust belts. Subduction zones are a special class of thrusts that form the largest faults on Earth and give rise to the largest earthquakes.

Oblique-slip faults

A fault which has a component of dip-slip and a component of strike-slip is termed an ''oblique-slip fault''. Nearly all faults have some component of both dip-slip and strike-slip; hence, defining a fault as oblique requires both dip and strike components to be measurable and significant. Some oblique faults occur within Shear (geology)#Transtension, transtensional and Shear (geology)#Transpression, transpressional regimes, and others occur where the direction of extension or shortening changes during the deformation but the earlier formed faults remain active. The ''hade'' angle is defined as the Angle#complementary angle, complement of the dip angle; it is the angle between the fault plane and a vertical plane that strikes parallel to the fault.

Listric fault

Listric faults are similar to normal faults but the fault plane curves, the dip being steeper near the surface, then shallower with increased depth. The dip may flatten into a sub-horizontal décollement, resulting in a horizontal slip on a horizontal plane. The illustration shows slumping of the hanging wall along a listric fault. Where the hanging wall is absent (such as on a cliff) the footwall may slump in a manner that creates multiple listric faults.

Ring fault

Ring faults, also known as caldera faults, are faults that occur within collapsed volcanic calderas and the sites of bolide strikes, such as the Chesapeake Bay impact crater. Ring faults are the result of a series of overlapping normal faults, forming a circular outline. Fractures created by ring faults may be filled by ring dikes.

Synthetic and antithetic faults

Synthetic and antithetic faults are terms used to describe minor faults associated with a major fault. Synthetic faults dip in the same direction as the major fault while the antithetic faults dip in the opposite direction. These faults may be accompanied by rollover anticlines (e.g. the Niger Delta Structural Style).

Fault rock

File:CREIGHTON-fault-sudbury-basin-science-north.jpg, upInactive fault from Greater Sudbury, Sudbury to Sault Ste. Marie, Ontario, Sault Ste. Marie, Northern Ontario, Canada All faults have a measurable thickness, made up of deformed rock characteristic of the level in the crust where the faulting happened, of the rock types affected by the fault and of the presence and nature of any Hydrothermal circulation, mineralising fluids. Fault rocks are classified by their Rock microstructure, textures and the implied mechanism of deformation. A fault that passes through different levels of the lithosphere will have many different types of fault rock developed along its surface. Continued dip-slip displacement tends to juxtapose fault rocks characteristic of different crustal levels, with varying degrees of overprinting. This effect is particularly clear in the case of detachment faults and major thrust faults. The main types of fault rock include: * Cataclasite – a fault rock which is cohesive with a poorly developed or absent planar Fabric (geology), fabric, or which is incohesive, characterised by generally angular Clastic rock, clasts and rock fragments in a finer-grained Matrix (geology), matrix of similar composition. ** Tectonic or fault breccia – a medium- to coarse-grained cataclasite containing >30% visible fragments. ** Fault gouge – an incohesive, clay-rich fine- to Ultrafine particles, ultrafine-grained cataclasite, which may possess a planar fabric and containing <30% visible fragments. Rock clasts may be present *** Clay smear - clay-rich fault gouge formed in Sedimentary rock, sedimentary sequences containing clay-rich layers which are strongly deformed and sheared into the fault gouge. * Mylonite – a fault rock which is cohesive and characterized by a well-developed planar fabric resulting from tectonic reduction of grain size, and commonly containing rounded porphyroclasts and rock fragments of similar composition to minerals in the matrix * Pseudotachylyte – ultrafine-grained glassy-looking material, usually black and flinty in appearance, occurring as thin planar Vein (geology), veins, injection veins or as a matrix to Conglomerate (geology), pseudoconglomerates or breccias, which infills dilation fractures in the host rock. Pseudotachylyte likely only forms as the result of seismic slip rates and can act as a fault rate indicator on inactive faults.

Impacts on structures and people

In geotechnical engineering, a fault often forms a Discontinuity (Geotechnical engineering), discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of soil and rock masses in, for example, tunnel, Foundation (engineering), foundation, or Slope stability analysis, slope construction. The level of a fault's activity can be critical for (1) locating buildings, tanks, and pipelines and (2) assessing the Seismic wave, seismic shaking and tsunami hazard to infrastructure and people in the vicinity. In California, for example, new building construction has been prohibited directly on or near faults that have moved within the Geologic time scale, Holocene Epoch (the last 11,700 years) of the Earth's geological history. Also, faults that have shown movement during the Holocene plus Pleistocene Epochs (the last 2.6 million years) may receive consideration, especially for critical structures such as power plants, dams, hospitals, and schools. Geologists assess a fault's age by studying soil features seen in shallow excavations and geomorphology seen in aerial photographs. Subsurface clues include shears and their relationships to Carbonate rock, carbonate Nodule (geology), nodules, Erosion, eroded clay, and Iron ore#Sources, iron Oxide minerals, oxide mineralization, in the case of older soil, and lack of such signs in the case of younger soil. Radiocarbon dating of Organic compound, organic material buried next to or over a fault shear is often critical in distinguishing active from inactive faults. From such relationships, paleoseismology, paleoseismologists can estimate the sizes of past earthquakes over the past several hundred years, and develop rough projections of future fault activity.

Faults and ore deposits

Many ore deposits lie on or are associated with faults. This is because the fractured rock associated with fault zones allow for magma ascent or the circulation of mineral-bearing fluids. Intersections of near-vertical faults are often locations of significant ore deposits. An example of a fault hosting valuable porphyry copper deposits is northern Chile's Domeyko Fault with deposits at Chuquicamata, Collahuasi, El Abra, Chile, El Abra, El Salvador mine, El Salvador, Escondida, La Escondida and Potrerillos Mine, Potrerillos. Further south in Chile Los Bronces mine, Los Bronces and El Teniente porphyry copper deposit lie each at the intersection of two fault systems.

See also

* Aseismic creep * * * * * *Paleostress inversion * * * Vertical displacement - Vertical movement of Earth's crust *Anderson's Theory of Faulting


* * * * * * * * * * * *

External links

Fault Motion Animations
at IRIS Consortium
Aerial view of the San Andreas fault in the Carrizo Plain, Central California, from "How Earthquakes Happen"
LANDSAT image of the San Andreas Fault in southern California, from "What is a Fault?"
at USGS {{DEFAULTSORT:Fault (Geology) Structural geology Stratigraphy Faults (geology) Tectonic landforms Earth's crust